In recent years, increasing the capacity of magnetic recording apparatus has been demanded and improvement in the recording density has been required since a magnetic disk of a small size and large capacity is mounted not only in personal computers but also in electric products for home use, for example. While development of magnetic heads, magnetic recording media, etc., has been conducted vigorously, the main stream of the recording system has been transited from the in-plane magnetic recording system to the perpendicular magnetic recording system and a further improvement of the recording density has become possible. In the perpendicular magnetic recording medium, it has been known that the efficiency of the recording magnetic field from the magnetic head can be improved by using a two-layered perpendicular magnetic recording medium having a soft magnetic underlayer and this can cope with increases in the coersivity of the recording film.
For the soft magnetic underlayer, a soft magnetic material of high saturation magnetic flux density (Bs) may be formed as compared with that of a recording layer in order to circulate the recording magnetic field from a magnetic head and, accordingly, noise due to the soft magnetic underlayer has caused a problem. The main cause of the noise is attributable to the leakage magnetic field from magnetic walls formed in the soft magnetic layer, and a method of suppressing the formation of the magnetic walls or a method of decreasing the leakage magnetic field has been studied so far.
For example, as disclosed in Japanese Patent Publication No. 7-129946 (“Patent Document 1”) and Japanese Patent Publication No. 11-191217 (“Patent Document 2”), there have been proposed a method of disposing a hard magnetic pinning layer between a soft magnetic underlayer and a substrate thereby aligning the magnetization of the soft magnetic underlayer in one direction, or a method of suppressing the movement of magnetic walls in the soft magnetic underlayer by exchange coupling with antiferromagnetism with the direction of the magnetic spins being aligned as disclosed in Japanese Patent Publication No. 6-103554 (“Patent Document 3”). Further, as disclosed in Japanese Patent Publication No. 2001-155321 (“Patent Document 4”), there has been proposed a method of constituting the soft magnetic underlayer with two or more soft magnetic layers separated from each other by a nonmagnetic layer and using Ru for the nonmagnetic layer thereby antiferromagnetically exchange coupling the two soft magnetic underlayers. The stacked soft magnetic layers aligned anti-parallel each other less generates leakage magnetic field from magnetic walls, spike noises or demagnetization of recording magnetization can be suppressed without a magnetic domain control layer and the robustness for stray magnetic field is also improved. Japanese Patent Publication No. 2005-302238 (“Patent Document 5”) describes that since the magnetic flux circulates between the layers by making the magnetic moment equal between the antiferromagnetically coupled two amorphous soft magnetic layers, this has a great effect for suppressing the spike noise and the amplitude modulation of reproduced signals, and this can provide an effect of improving the robustness against stray magnetic field by providing a uniaxial anisotropy in the radial direction of the first amorphous soft magnetic layer and the second amorphous soft magnetic layer to decrease the coersivity. Further, Patent Document 5 describes that the antiferromagnetic coupling exerting between amorphous soft magnetic layers can be strengthened by making the nonmagnetic layer into a sandwich structure put between Co series ferromagnetic layers, or by using RuCo or RuFe for the nonmagnetic layer.
As described above, the material used for soft magnetic underlayer is not particularly restricted so long as it has high Bs, is provided with a uniaxial anisotropy in the radial or circumferential direction of a disk substrate, and excellent in the surface planarity, and amorphous alloys comprising Co or Fe as the main ingredient have been used so far. However, a problem that the soft magnetic underlayer of a perpendicular magnetic recording medium suffers from corrosion has been arisen recently and it has been found that corrosion tends to occur particularly in a case of using Co alloys. Since Co alloys are not excellent in the corrosion resistance and have an extremely basic potential in the circumstance of an aqueous solution, they suffer from galvanic corrosion (corrosion between different kinds of metals) relative to an adjacent intermediate layer. Since Ru or Ru alloys well known as the intermediate layer has an extremely high potential because they are noble metal and the potential difference between both of them reaches as high as about 1.0 V, corrosion of the Co alloy is extremely accelerated by the galvanic corrosion than the corrosion for a single element. On the other hand, Fe series alloys have a relatively high potential although their corrosion resistance is not so different from that of the Co alloys and scarcely cause galvanic corrosion even when they are in adjacent with Ru.
As described above, a method of separating the two soft magnetic layers by the Ru nonmagnetic layer and antiferromagnetically exchange-coupling them is extremely effective in that noise due to the soft magnetic underlayer is eliminated. The optimized thickness of the Ru nonmagnetic layer for maximizing the exchange-coupling of the soft magnetic layer is different depending on the material of the soft magnetic layer. In a case of using a Co alloy for the soft magnetic layer, the optimized film thickness of the Ru is about 1 nm, whereas the optimized film thickness as thin as 0.4 nm in a case of using an Fe alloy material. Further, in a case of using the Fe alloy material for the soft magnetic layer, since the dependence of the exchange-coupled magnetic field on the thickness of the Ru film is large compared with the case of using the Co alloy as to be described later, the margin for the film thickness is narrow and control thereof is difficult in view of the mass production.